EUV light, which is defined as electromagnetic radiation with wavelengths between 124 nm and 10 nm, is used in next-generation photolithography systems to produce structures smaller than is possible with current ultra-violet light sources, such as excimer lasers. One method of creating EUV light involves spinning a cylinder with an external surface coated with a uniformly thick layer of solid xenon and hitting the xenon surface with a pulsed laser of sufficient intensity to create a plasma. Prior to the next pulse of light, the cylinder is rotated and/or translated to expose a fresh region of solid xenon. As the cylinder rotates, gaseous xenon is continuously sprayed onto the cold surface of the cylinder, reforming the frozen xenon surface over time. A particular spot or zone cannot be hit again until sufficient time has elapsed for the solid xenon surface to reform and return to its original condition. Adjacent spots must be separated by a minimum distance until the solid xenon is fully reformed to prevent damage to xenon ice, causing it to delaminate from the rotating cold cylinder.
As EUV light is strongly absorbed by many substances, the plasma is created in a vacuum chamber kept at a low ambient pressure by a vacuum pump. Outgassing of substances from the equipment within the vacuum chamber or other contamination of the vacuum chamber is undesirable, as it reduces the efficiency of the EUV light source by absorbing the EUV light as it is generated. Xenon and argon attenuate EUV light less than other substances at a given pressure, so if it necessary or unavoidable to have small amounts of some substance enter the vacuum chamber, these contaminants should be xenon or argon.
Due to the need to accurately target the solid xenon layer on the surface of the cylinder, the cylinder must be able to rotate with high precision and very low mechanical vibration levels and uncertainty of position. Additionally, the cylinder must be able to be filled with liquid nitrogen to reduce its temperature below the freezing point of xenon. Rotation bearings, preloaded mechanical ball bearings, and axial mechanical thrust bail bearings can be used to support rotation of the cylinder with ferrofluidic bearings used to effect a vacuum seal between the cylinder apparatus and the vacuum chamber. However, these mechanisms have certain disadvantages when used in an EUV light source, including creating unacceptably high numbers of particles through mechanical wear of the bearings, outgassing of lubricating substances into the vacuum chamber, and a tendency to cause significant vibrations and alignment stress to the rotating elements. Ferrofluidic bearings are complex, also prone to outgassing, and thus contamination of the vacuum chamber, and cause further mechanical alignment stress to the rotating elements.
Gas bearings have several advantages over mechanical bearings, as support essentially friction-free and contact-free rotation and axial motion, and they do not have the mechanical wear and lubrication outgassing problems of mechanical bearings. However, even with a gas bearing, the seal between the rotating spindle upon which the target cylinder is disposed and the stator body that supports the spindle is not perfect and the gas used in the gas bearing can enter the vacuum chamber, attenuating the EUV light as it is generated.